1. Field of Invention
This invention relates to a system and method of operating a measuring cell of an ion cyclotron resonance (ICR) mass spectrometer (MS), preferably of a Fourier transform ICR (FTICR) MS.
2. Discussion of the Background
In an ion cyclotron resonance mass spectrometer, the mass-specific cyclotron motions of the ions in a magnetic field are detected as image currents induced by the ions in detection electrodes. Typically, in ICR mass spectrometers, the detection of fundamental frequencies of ion oscillations is performed.
Currently, the time of analysis and its sensitivity become the most valuable analytical parameters for ICR mass spectrometers. The main contribution to the analysis time is the duration T of the acquired transient signal itself. The minimal T is determined by the desired resolving power R for ions of a specified mass-to-charge ratio value m/z having measured cyclotron frequency ω+:T=4πR/ω+  (1)Since ω+ is proportional to the strength of the magnetic field B of ICR mass spectrometer, the minimal required transient duration T seems to be limited by the magnetic field B. To overcome this limitation, it was suggested in the 1980s to use multiple-electrode detection plate arrangements. In multiple-electrode arrangements, each of the detection electrodes is split into several smaller electrodes. These electrodes are connected to an amplifier of the image signal in such a way that the detection of the ion oscillation overtone frequencies is performed. The overtone frequencies typically occur on multiples of the ion cyclotron frequency ω+, i.e. the overtone frequencies have frequencies Mω+, where M is an integer. When the decoherence time of the ion cloud with excited coherent cyclotron motion exceeds a duration of the acquired transient T, the multiple-electrode cell gives the improvement M in the obtained resolving power predicted by Eqn. (1). Alternatively, the same resolving power R is obtained with M times shorter transient.
A number of multiple-electrode cell designs have been suggested. Their common drawback is the reduced sensitivity compared to the conventional cell designs. To obtain the same sensitivity, the ion cyclotron radius in a multiple-electrode cell has to be larger for larger values of the frequency multiple M. Excitation of ions to the orbits larger than half of the cell radius is not always a desirable condition in ICR experiments. Among the reasons for this undesirability are 1) deviation of the trapping potential from the quadrupolar form in cylindrical and cubic cells at large radii and 2) possible dephasing of the ion cloud during excitation to large orbits. For a given radius r of excitation in a conventional cell with one pair of detection electrodes and a multiple-electrode cell of the same radius R having M pairs of detection electrodes, the intensity of the signal obtained in the latter cell is (r/R)M-1 times the intensity in the former one. Given that r/R<1, the difference in signal intensities is considerable at a small excitation radii.
An “O-trap” design (known in the art) addressed the speed of analysis issue in FTICR mass spectrometry in general and the sensitivity issues of the conventional multiple-electrode FTICR cells in particular. The “O-trap” concept includes separating the functions of ion excitation and detection between two different FTICR cell compartments. The “detection” compartment of the “O-trap” (where detection of the ion motion is performed) implements additional internal coaxial electrodes around which ions with excited cyclotron motion revolve. The separation of excitation and detection functions facilitates implementation of versatile techniques unattainable in a single compartment of a conventional FTICR cell (including prior-art multiple-electrode cells).
The following references (incorporated by reference herein in their entirety) describe this background technology:    1. Marshall A. G., Hendrickson C. L., Jackson G. S.; Mass Spectrom. Rev. 1998; 17: 1.    2. Amster J.; J. Mass Spectrom. 1996; 31: 1325.    3. Nikolaev E. N., et al.; USSR Inventor's Certificate SU1307492, 1985.    4. Nikolaev E. N., et al.; USSR Inventor's Certificate SU1683841, 1989.    5. Nikolaev E. N., et al.; Rapid Commun. Mass Spectrom. 1990; 4: 144-146.    6. Rockwood A., et al.; U.S. Pat. No. 4,990,775, 1991.    7. Pan Y., Ridge D. P., Rockwood A. L.; Int. J. Mass Spectrom. Ion Processes 1988; 84: 293.    8. Misharin A. S., Zubarev R. A.; Proc. 54th ASMS Conference, Seattle, Wash., 2006, Session: Instrumentation—FTMS—210.    9. Misharin A. S., Zubarev R. A.; Rapid Commun. Mass Spectrom. 2006; 20: 3223-3228.    10. Knobeler M., Wanczek K. P.; Proc. of the 45th ASMS conference, Palm Springs, Calif., 1997; p. 864.    11. Kaiser N. K., Bruce J. E.; International Journal of Mass Spectrometry 2007; 265(2-3): 271-280.    12. Weisbrod C. R., et al.; Anal. Chem. 2008; 80(17): 6545-6553.    13. Kim S., et al.; Anal. Chem. 2007; 79(10): 3575-3580.    14. Caravatti P., Allemann M.; Org. Mass Spectrom. 1991; 26: 514-518.    15. Lammert S. A., et al.; International Journal of Mass Spectrometry 2001; 212(1-3): 25-40.    16. Lammert S. A. et al.; J. Am. Soc. Mass Spectrom. 2006; 17: 916-922.    17. Tolmachev A. V., et al.; J. Am. Soc. Mass Spectrom. 2008; 19(4): 586-597.    18. Brustkern A. M. et al.; J. Am. Soc. Mass Spectrom. 2008; 19: 1281-1285.    20. Gorshkov M. V., Pa{hacek over (s)}a-Tolić L., Bruce J. E., Anderson G. A., Smith R. D. Anal. Chem. 1997, 69, 1307-1314    21. McIver R. T. et al.; International Journal of Mass Spectrometry and Ion Processes 1985 (64), 67.    22. M. V. Gorshkov et al., Journal of the American Society for Mass Spectrometry, 2001 (12), 1169.    23. Guan S., Marshall A. G. International Journal of Mass Spectrometry and Ion Processes 1995, 146/147: 261-296.    24. Misharin A. S., Zubarev R. A., Doroshenko V. M., In: Proc. 57th ASMS Conference, Philadelphia, Pa., 2009, Session: Instrumentation—FTMS—285